A field-effect transistor is one of the most widely-used semiconductor elements. A variety of materials are used for field-effect transistors according to their uses. In particular, semiconductor materials including silicon are frequently used.
A field-effect transistor including silicon has characteristics satisfying the needs for a variety of uses. For example, single crystal silicon is used for an integrated circuit or the like which needs to be operated at a high speed, whereby the need is satisfied. Further, amorphous silicon is used for an object which needs to have a large area, such as a display device, whereby the need can be satisfied.
As described above, silicon is highly versatile and can be used for various purposes. However, in recent years, semiconductor materials have come to be expected to have higher performance as well as versatility. For example, in terms of improving performance of a large-area display device, in order to realize high-speed operation of a switching element, a semiconductor material which facilitates the increase in area of a display device and shows higher performance than amorphous silicon is needed.
Under such conditions, a technique for a field-effect transistor (also called an FET) including an oxide semiconductor has attracted attention. For example, Patent Document 1 discloses a transparent thin film field-effect transistor including a homologous compound InMO3(ZnO)3(M is In, Fe, Ga, or Al, and m is an integer greater than or equal to 1 and less than 50).
In addition, Patent Document 2 discloses a field-effect transistor in which an amorphous oxide semiconductor which contains In, Ga, and Zn and has an electron carrier density less than 1018/cm3 is used. Note that in this patent document, the ratio of In atoms to Ga atoms and Zn atoms in the amorphous oxide semiconductor is 1:1:m (m<6).
Further, Patent Document 3 discloses a field-effect transistor in which an amorphous oxide semiconductor including a microcrystal is used for an active layer.